Surfactant Aggregation at a Hydrophobic Surface

Effect of Solute on Interfacial Properties and Micelle Structure of Dodecylbenzenesulfonate (DBS): Experimental and Molecular Dynamics Studies

A combined experimental and molecular dynamic simulation approach was used to examine the structure and interfacial properties of solute-saturated micelles. The properties of dodecylbenzenesulfonate (DBS) micelles were examined in dodecane and benzene hydrocarbon systems. Pyrene fluorescence was used to determine the aggregation number of surfactant monomers in the micelle systems. Molecular dynamic (MD) simulations using energy minimization applying the CHARMm force field with the TIP3P model for water. Comparison of the DBS/benzene and DBS/Dodecane micelles equilibrium structures via radial distribution function (RDF) and probability distribution function (PDF) analysis indicates that the area per head group for the DBS/Benzene micelle interface is significantly larger than that of the DBS/Dodecane at the interface. It was also determined that benzene molecules can move freely within the micelle while dodecane is strictly confined in the core of the micelle. The increased interfacial area per monomer caused by the insertion of benzene also reduces the effectiveness of the surfactant, which has implications for use in various environmental applications. However, the DBS/benzene micelle can solubilize many more hydrocarbon molecules in one micelle with less surfactant monomer (i.e., lower aggregation number) per micelle due to the increased available packing positions within the micelle. This, in turn, increases the efficiency of the surfactant in real-world applications which is consistent with previous laboratory results. Understanding the differing solubilization characteristics of surfactants against various classes of hydrocarbons in single solute systems is a necessary step to beginning to understand their solubilization properties in the mixed waste systems prevalent in most surfactant enhanced remediation (SEAR) strategies.

Adsorption of Photoresponsive Surfactants at Solid-Liquid Interfaces

We report on the adsorption kinetics of azobenzene-containing surfactants on solid surfaces of different hydrophobicity. The understanding of this processes is of great importance for many interfacial phenomena that can be actuated and triggered by light, since the surfactant molecules contain a photoresponsive azobenzene group in their hydrophobic tail. Three surfactant types are studied, differing in the spacer connecting the headgroup and the azobenzene unit by between 6 and 10 CH₂ groups. Under irradiation with light of a suitable wavelength, the azobenzene undergoes reversible photoisomerization between two states, a nonpolar trans-state and a highly polar cis-state. Consequently, the surfactant molecule changes its hydrophobicity and thus affinity to a surface depending on the photoisomerization state of the azobenzene. The adsorption behavior on hydrophilic (glass) and hydrophobic (TeflonAF) surfaces is analyzed using quartz crystal microbalance with dissipation (QCM-D) and ζ-potential measurements. At equilibrium, the adsorbed surfactant amount is almost twice as large on glass compared to TeflonAF for both isomers. The adsorption rate for the trans-isomers on both surfaces is similar, but the desorption rate of the trans-isomers is faster at the glass-water interface than at the Teflon-water interface. This result demonstrates that the trans-isomers have higher affinity for the glass surface, so the trans-to-cis ratios on glass and TeflonAF are 80/1 and 2/1, respectively, with similar trends for all three surfactant types.

Electrodeposition-Assisted Rapid Preparation of Pt Nanocluster/3D Graphene Hybrid Nanozymes with Outstanding Multiple Oxidase-Like Activity for Distinguishing Colorimetric Determination of Dihydroxybenzene Isomers

Here, we demonstrate a facile bottom-up strategy to fabricate Pt nanoclusters (Pt NCs) grafted onto three-dimensional graphene foam (3D GF) assisted by cetyltrimethyl ammonium bromide (CTAB) using the electrodeposition method. The homogeneous grafting of Pt NC onto 3D GF is due to the formation of hemimicelles above some CTAB concentration. With the unique nanocluster structure and the high content of Pt⁰, the Pt NC/3D GF nanohybrid exhibits extremely high activity and shows higher reusability and stability. Apart from the intrinsic oxidase-like activity with 3,3',5,5'-tetramethylbenzidine (TMB) as the substrate, the Pt NC/3D GF nanohybrid can act simultaneously as an effective polyphenol oxidase (PPO) mimic, such as tyrosinase, catechol oxidase, and laccase. More importantly, utilizing intrinsic catechol oxidase-like activity and the oxidase-like activity with TMB as the substrate of the nanohybrid, distinguishing colorimetric determination of dihydroxybenzene isomers (catechol and hydroquinone) is performed. Distinguishing colorimetric analysis of dihydroxybenzene isomers was first developed using nanozymes. The present work provides a simple bottom-up approach for the reasonable fabrication of various nanostructured nanozymes with excellent performance using the electrodeposition method assisted with surfactants.

Inhomogeneous Distribution of Cationic Surfactants around Anionic Molecular Clusters

An interesting phenomenon is reported when uranyl peroxide nanoclusters U₆₀ (Li_48+m K₁₂ (OH)_m [UO₂ (O₂ )(OH)]₆₀ (H₂ O)_n , m≈20 and n≈310) interact with a small number of cationic surfactant molecules. Cationic surfactant molecules do not distribute evenly around the U₆₀ clusters during the interaction as expected. Instead, a small fraction of U₆₀ clusters attract almost all the surfactant molecules, leading to the self-assembly into supramolecular structures by using surfactant-U₆₀ complexes as building locks, and later further aggregate and precipitate based on hydrophobic interaction, whereas the rest of the clusters remained unbounded soluble macroions in bulk dispersion. This phenomenon nicely demonstrates a unique feature of macroion solutions. Considering that Debye-Hückel approximation is no longer valid in such solutions, the competition between the local electrostatic interaction and hydrophobic interaction becomes important to regulate the solution behaviors of macroions.

Electrochemical exfoliation of graphite in nanofibrillated kenaf cellulose (NFC)/surfactant mixture for the development of conductive paper

The effect of incorporating common dodecyl anionic and cationic surfactants such as dodecyltrimethylammonium bromide (DTAB), dodecylethyldimethylammonium bromide (DDAB), and sodium dodecylsulfate (SDS) in nanocomposites of reduced graphene oxide and nanocellulose are described. The stabilization and electrical properties of the nanocomoposites of reduced graphene oxide (RGO) and nanofibrillated kenaf cellulose (NFC) were characterized using four-point probe electrical conductivity measurements. Raman spectroscopy, field emission scanning electron microscopy, and high-resolution transmission electron microscopy were used to investigate dispersion morphology and the quality of RGO inside the NFC matrices. Small-angle neutron scattering (SANS) was used to study the aggregation behavior of the aqueous surfactant systems and RGO dispersions. The cationic surfactant DTAB proved to be the best choice for stabilization of RGO in NFC, giving enhanced electrical conductivity five orders of magnitude higher than the neat NFC. The results highlight the effects of hydrophilic surfactant moieties on the structure, stability and properties of RGO/NFC composites.

Comparing Surfactant Structures at "Soft" and "Hard" Hydrophobic Materials: Not All Interfaces Are Equivalent

The interfacial structures of a range of amphiphilic molecules are studied with both "soft" and "hard" hydrophobic substrates. Neutron reflection and quartz crystal microbalance with dissipation measurements highlight the differences between the adsorbed structures adopted by sodium dodecyl sulfate (SDS), cetyltrimethylammonium bromide (C₁₆TAB), and the "AM1" surface active peptide. At the soft siloxane/water interface, small molecular surfactants form loosely packed layers, with the hydrophobic tails penetrating into the oily layer, and an area per surfactant molecule that is significantly less than previously reported for the air/water interface. Neutron reflection measurements, supported by quartz crystal microbalance studies, indicate that for C₁₆TAB, approximately 30 ± 8% of the alkyl tail penetrates into the poly(dimethylsiloxane) (PDMS) layer, whereas 20 ± 5% of the alkyl tail of SDS is located in the PDMS. For the engineered peptide surfactant AM1 (21 residues), it was found that one face of the α helix penetrated into the PDMS film. In contrast, penetration of the surfactant tails was not observed against hard solidlike hydrophobic surfaces made from octadecyltrichlorosilane (OTS) for any of the molecular species studied. At the OTS/water interface, C₁₆TAB and SDS were seen to adsorb as larger aggregates and not as monolayers. Amphiphilic adsorption (amount, structural conformation) at the PDMS/water interface is shown to be different from that at both the air/water interface and the hard OTS/water interface, illustrating that interfacial structures cannot be predicted by the surfactant packing parameter alone. The bound PDMS layer is shown to be a useful proxy for the oil/water interface in surface and stabilization studies, with hydrophobic components of the molecules able to penetrate into the oily PDMS.

Ice-like water supports hydration forces and eases sliding friction

The nature of interfacial water is critical in several natural processes, including the aggregation of lipids into the bilayer, protein folding, lubrication of synovial joints, and underwater gecko adhesion. The nanometer-thin water layer trapped between two surfaces has been identified to have properties that are very different from those of bulk water, but the molecular cause of such discrepancy is often undetermined. Using surface-sensitive sum frequency generation (SFG) spectroscopy, we discover a strongly coordinated water layer confined between two charged surfaces, formed by the adsorption of a cationic surfactant on the hydrophobic surfaces. By varying the adsorbed surfactant coverage and hence the surface charge density, we observe a progressively evolving water structure that minimizes the sliding friction only beyond the surfactant concentration needed for monolayer formation. At complete surfactant coverage, the strongly coordinated confined water results in hydration forces, sustains confinement and sliding pressures, and reduces dynamic friction. Observing SFG signals requires breakdown in centrosymmetry, and the SFG signal from two oppositely oriented surfactant monolayers cancels out due to symmetry. Surprisingly, we observe the SFG signal for the water confined between the two charged surfactant monolayers, suggesting that this interfacial water layer is noncentrosymmetric. The structure of molecules under confinement and its macroscopic manifestation on adhesion and friction have significance in many complicated interfacial processes prevalent in biology, chemistry, and engineering.

Electrolyte-induced reorganization of SDS self-assembly on graphene: a molecular simulation study

A molecular dynamics simulation was conducted to study the structure and morphology of sodium dodecyl sulfate (SDS) surfactants adsorbed on a nanoscale graphene nanostructure in the presence of an electrolyte. The self-assembly structure can be reorganized by the electrolyte-induced effect. An increase in the ionic strength of the added electrolyte can enhance the stretching of adsorbed surfactants toward the bulk aqueous phase and make headgroups assemble densely, leading to a more compact structure of the SDS/graphene composite. The change in the self-assembly structure is attributed to the accumulation/condensation of electrolyte cations near the surfactant aggregate, consequently screening the electrostatic repulsion between charged headgroups. The role of the electrolyte revealed here provides direct microscopic evidence or an explanation of the reported experiments in the electrolyte tuning of the interfacial structure of a surfactant aggregate on the surface of carbon nanoparticles. Additionally, the buoyant density of the SDS/graphene assembly has been computed. With an increase in the ionic strength of the electrolyte, the buoyant density of the SDS/graphene composite rises. The interfacial accumulation of electrolytes provides an important contribution to the density enhancement. The study will be valuable for the dispersion and application of graphene nanomaterials.

Growing nano-petals on electrospun micro/nano fibers

Novel architectures: growing petals on micro/nano fibers were fabricated via combining electrospinning with solvent-induced self-assembly.

Monte Carlo study of the adsorption and aggregation of alkyltrimethylammonium chloride on the montmorillonite-water interface

Organically modified clays exhibit adsorption capacities for cations, anions, and nonpolar organic compounds, which make them valuable for various environmental technical applications. To improve the understanding of the adsorption processes, the molecular-scale characterization of the structures of organic aggregates assembled on the external basal surfaces of clay particles is essential. The focus of this Monte Carlo simulation study was on the effects of the surface coverage and the alkyl chain length n on the structures of alkyltrimethylammonium chloride ((C(n)TMA)Cl) aggregates assembled on the montmorillonite-water interface. We found that the amount of adsorbed C(n)TMA(+) ions is independent of the alkyl chain length and increases with the C(n)TMA(+) surface coverage. The C(n)TMA(+) ions predominantly adsorb as inner-sphere complexes; the fraction of outer-sphere adsorbed ions equals only about 10%. The conformational order of the C(n)TMA(+) alkyl chains substantially decreases with decreasing alkyl chain length. In agreement with previous experiments, the amount of C(n)TMA(+) ions that are aggregated at the mineral surface increases with increasing chain length. The maximum value of 0.66 C(n)TMA(+) adsorption complex per unit cell area of the clay surface considerably exceeds the amount of cations required to compensate the negative charge of the montmorillonite surface. Furthermore, in most of the studied systems, fractions of Na(+) surface cations remain adsorbed on montmorillonite. The resulting interfacial positive charge excess is counterbalanced by coadsorbed chloride ions forming ion pairs with both C(n)TMA(+) and Na(+).

Aggregation of alkyltrimethylammonium ions at the cleaved muscovite mica-water interface: a Monte Carlo study

The precise molecular structure of organically modified mineral surfaces is still not well understood. To establish a relation between experimental observations and underlying molecular structure, we performed Monte Carlo simulations of the aggregation behavior of alkyltrimethylammonium surfactants (C(n)TMA(+)) at the interface between C(n)TMACl solution and cleaved K(+)-muscovite. The structures were examined with regard to the influence of varying alkyl chain length n (n = 8, 12, 16) and surface coverage of C(n)TMA(+) ions. The simulation results indicate that the water film structure at the muscovite surface is considerably influenced by the adsorption of C(n)TMA(+). A fraction of the C(n)TMA(+) ions forms inner-sphere and outer-sphere adsorption complexes with nitrogen-surface distances of 3.3-3.8 and 5.5-8.4 Å, respectively. The simulated monolayer aggregates exhibit thicknesses of 31-35, 22-27, and ∼18 Å for C(16)TMA(+), C(12)TMA(+), and C(8)TMA(+), respectively. C(16)TMA(+) and C(12)TMA(+) ions form bilayer aggregates, which show a strong interdigitation of the two opposing layers composing them. The aggregate thicknesses equal 35-39 and 30-35 Å, respectively, and are in agreement with available experimental data. In contrast, the short-chained C(8)TMA(+) ions do not form bilayer aggregates. In agreement with previous experimental studies, the alkyl chains of the aggregated ions show high conformational order markedly decreasing with decreasing chain length. We suggest that the simulated structures represent C(n)TMA(+) aggregates, which are formed on muscovite during the experimentally observed initial equilibration phase characterized by the presence of inorganic ions within the aggregates.

Calculations of the charge distribution in dodecyltrimethylammonium: a quantum chemical investigation

In this work we report the atomic partial charges evaluated on dodecyltrimethylammonium ion. The values obtained from 17 quantum methods [CHELP, CHELPG, MK, NPA at (HF, LDA, PBE, B3LYP)//6-31G++(d,p) level and APT at B3LYP//6-31G++(d,p)] on the molecule optimised at B3LYP/6-31G++(d,p) level were compared to two semiempirical methods (Gasteiger and Qeq) and the commercial force field PCFF. All methods based on quantum calculation gave a positive charge delocalised on at least the first four alkyl groups of the tail. However, those deriving partial charges from the electrostatic potential gave an unrealistic set of alternative positive and negative alkyl group charges along the tail. In comparison, the NPA and APT methods lead to a steady decrease in the partial charges from the third alkyl group, and agreed closely with the representation of the electrostatic potential mapped onto the 0.002 au isodensity surface. The choice of the exchange correlation treatment does not drastically influence the atomic partial charges.

Interaction of cationic hydrophobic surfactants at negatively charged surfaces investigated by atomic force microscopy

Atomic force microscopy was used to study the adsorption of the surfactant octadecyl trimethyl ammonium chloride (C18TAC) at a low concentration (0.03 mM) to negatively charged surfaces in water. Atomic force microscopy tips were functionalized with dimethyloctadecyl(3-tripropyl)ammonium chloride (C18TAC-si) or N-trimethoxysilylpropyl-N,N,N-trimethylammomium chloride (hydrophilpos-si) to facilitate imaging of the adsorbed surfactant without artifacts. Tapping mode images and force measurements revealed C18TAC patches, identified as partial surfactant bilayers or hemimicelles. The forces controlling the adsorption process of the C18TAC to a negatively charged surface were investigated by measuring the forces between a C18TAC-si or a hydrophilpos-si tip and a silica surface in the presence of varying concentrations of either NaCl or NaNO3. Screening of forces with an increasing NaCl concentration was observed for the C18TAC-si and hydrophilpos-si tips, proving an electrostatic contribution. Screening was also observed for the hydrophilpos-si tip in NaNO3, whereas a long-range attraction was observed for the C18TAC-si tip for all NaNO3 concentrations. These results indicate that screening of the forces for the C18TAC-si tip depended on the type and/or size of the anion, possibly due to a different probability of the anions to enter the silane layers. The interaction of C18TAC patches with C18TAC-si tips in the presence of NaCl and the interaction of the patches with hydrophilpos-si tips in either NaCl or NaNO3 were repulsive and independent of the number of force curves measured, indicating a stable, positively charged C18TAC patch. However, the forces measured between the patches and a C18TAC-si tip in NaNO3 depended on the number of force curves measured, indicating a change in patch structure induced by the first interaction.

Structure of the sodium dodecyl sulfate surfactant on a solid surface in different NaCl solutions

Studies of molecular dynamics simulations of sodium dodecyl sulfate (SDS) molecules adsorbed on a graphite surface in different salt (NaCl)/water solutions were conducted. The results showed the formation of hemicylindrical aggregates, at different salt concentrations, in agreement with atomic force microscopy (AFM) results. However, the hemicylinders exhibited different structures as the salt concentration was increased. At low concentrations, the internal structure of the hemicylinder formed well-defined SDS layers, parallel to the surface. However, when the amount of salt was increased, the top layer became less pronounced until it disappeared at the highest concentration. Density profiles of the SDS headgroups were also analyzed, and those profiles were found to become sharper as the NaCl concentration increased. The phenomenon was investigated in terms of how the aggregates wet the solid surface.

SDS surfactants on carbon nanotubes: aggregate morphology

Although carbon nanotubes have attracted enormous research interest, their practical application is still hindered, primarily, by the difficulty of separating them into samples monodispersed in diameter, chirality, and length. Recent advances show that ultracentrifugating carbon nanotube dispersions stabilized by surfactants is a promising route for achieving the desired separation. For further perfectioning this procedure it is necessary to know how surfactants adsorb on nanotubes of different diameters, which determines the nanotube-surfactant aggregate effective density and the nanotube-nanotube potential of mean force. Because only limited experimental data are available to elucidate these phenomena, we report here an extensive all-atom molecular dynamics study on the morphology of sodium dodecyl sulfate (SDS) surfactant aggregates adsorbed on (6,6), (12,12), and (20,20) single walled carbon nanotubes at room conditions. Our calculations reveal that the nanotube diameter is the primary factor that determines the morphology of the aggregates because of a competition between the entropic and energetic advantage encountered by the surfactants when they wrap one nanotube, and the enthalpic penalty faced during this process due to bending of the surfactant molecule. The data are in qualitative agreement with the neutron scattering results reported by Yurekli et al. [J. Am. Chem. Soc. 2004, 126, 9902], and for the first time provide an atomic-level description helpful in designing better separation, as well as stabilization techniques for aqueous carbon nanotube dispersions.

Comparison between inhomogeneous adsorption of charged surfactants on air-water and on solid-water interfaces by self-consistent field theory

We use a realistic molecular model to study the interfacial behavior of hydrocarbon sulfate surfactants within a self-consistent field model and consider the adsorption both at the air-water interface and at a hydrophobic solid-water interface. We focus on the structural properties of the hemimicelles at the critical interface aggregation concentration (CIAC) for the air-water system and the critical surface aggregation concentration (CSAC) for the solid-water system. The major difference between the two systems is that the liquid interface is penetrable but the solid surface is intrinsically impenetrable for the molecular species. At the LG interface the hemimicelles have a lens shape with their centers of mass positioned slightly toward the aqueous side and feature an aspect ratio of approximately 2, with the long dimension parallel to the interface. Hemimicelle formation occurs below a critical (interfacial) area per molecule and above a critical surface pressure depending on tail length and ionic strength. Hemimicelles are not expected at air-water interfaces for a surfactant with a tail length ( t) lower than 15 CH2 units. In contrast, at a hydrophobic solid the formation of laterally inhomogeneous micelles even takes place for surfactants with the tail length as short as t = 12. This difference is attributed to the screening of the lateral interactions in the vapor phase. The shape of surface hemimicelles is caplike (or half-lens) with an aspect ratio lower than 2 and the long dimension parallel to the solid surface. The tail length, the ionic strength, the adsorption energies, and the surfactant concentration have an effect on the surface micelle properties such as the aggregation number and size and shape.

Conducting film from graphite oxide nanoplatelets and poly(acrylic acid) by layer-by-layer self-assembly

A [poly(acrylic acid)/graphite oxide]n [(PAA/GO)(n)] film with a conductivity of 60 S.cm(-1) was grown layer-by-layer (LbL) using Langmuir-Blodgett self-assembly techniques. GO nanoplatelets were prepared from natural graphite by oxidizing, ball milling, exfoliating, and modifying with cationic surfactant cetyltrimethylammonium bromide (CTAB). The X-ray diffraction pattern reveals that PAA and GO stack orderly LbL and repeatedly in the (PAA/GO)(n) films, and about three carbon molecular layers are superposed on each GO sheet. Fourier transform infrared spectra offer evidence for the interaction between the carboxylic groups on PAA and the CTAB on the surface of the GO nanoplatelets. Electrochemistry measurements show that the conductivity of the (PAA/GO)(n) film depends on the carbon-carbon interlayer height of the GO sheet, and the (PAA/GO)(n) film has a typical positive temperature coefficient effect above the PAA melting temperature. The atomic force microscopy images reveal that CTAB molecules stack in a well-ordered head-to-head structure on both surfaces of the GO nanoplatelets and the GO nanoplatelets are embeded between PAA layers.

Adsorption and self-assembly of surfactant/supercritical CO2 systems in confined pores: a molecular dynamics simulation

A coarse-grained molecular dynamics simulation has been carried out to study the adsorption and self-organization for a model surfactant/supercritical CO2 system confined in the slit-shape nanopores with amorphous silica-like surfaces. The solid surfaces were designed to be CO2-philic and CO2-phobic, respectively. For the CO2-philic surface, obviously surface adsorption is observed for the surfactant molecules. The various energy profiles were used to monitor the lengthy dynamics process of the adsorption and self-assembly for surfactant micelles or monomers in the confined spaces. The equilibrium properties, including the morphologies and micelle-size distributions of absorbed surfactants, were evaluated based on the equilibrium trajectory data. The interaction between the surfactant and the surface produces an obvious effect on the dynamics rate of surfactant adsorption and aggregation, as well as the final self-assembly equilibrium structures of the adsorbed surfactants. However, for the CO2-phobic surfaces, there are scarcely adsorption layers of surfactant molecules, meaning that the CO2-phobic surface repels the surfactant molecules. It seems to conclude that the CO2 solvent depletion near the interfaces determines the surface repellence to the surfactant molecules. The effect of the CO2-phobic surface confinement on the surfactant micelle structure in the supercritical CO2 has also been discussed. In summary, this study on the microscopic behaviors of surfactant/Sc-CO2 in confined pores will help to shed light on the surfactant self-assembly from the Sc-CO2 fluid phase onto solid surfaces and nanoporous media.

Self-Aggregation of the SDS Surfactant at a Solid−Liquid Interface

Molecular dynamics simulations of sodium dodecyl sulfate (SDS) molecules on a graphite surface are presented. The simulations were conducted at low and high surface coverage to study aggregation at the water/graphite interface. Results showed that at low surface coverage, the SDS molecules form hemicylindrical aggregates, in agreement with AFM experiments, whereas at high surface coverage, the surfactants form full cylinders. The latter aggregates have not been reported in systems of SDS on hydrophobic substrates, such as graphite. The unexpected results are explained in terms of a water layer adsorbed at the solid surface which was the responsible for the formation of these aggregates. Moreover, the SDS tails in the full cylindrical configuration became straighter than those of the hemicylindrical aggregate. Hydrogen bond formation between water and surfactant head groups was also studied, and it was found that they did not depend on the surfactant concentration.

Adsorption and morphology transition of surfactants on hydrophobic surfaces: a lattice Monte Carlo study

In this work, we first show that there are only five independent interchange parameters in the surfactant-solvent-interface system in Larson's model, and then adsorption and morphology transition of surfactants on hydrophobic surfaces are studied by extensive lattice Monte Carlo simulations. In our simulations, we found that there exist six adsorbed morphologies: (1) premature admicelle, (2) hemisphere, (3) hemisphere-hemicylinder mixture, (4) wormlike hemicylinder, (5) perforated monolayer, and (6) monolayer. The surface morphologies and the amount of adsorption on hydrophobic surfaces are found to be affected obviously by two interchange parameters. One is the attractive interaction between tail groups and surface (chiTS), and the other is the solubility of head groups in bulk (chiHW). Phase diagrams in chiHW versus chiTS planes for surfactants with different hydrophobicities (chiTW) and for surfactants with different molecular structures are determined in this work, from which the transitions of surface morphologies and adsorption behaviors are discussed.

Gemini Surfactants at the Air/Water Interface: A Fully Atomistic Molecular Dynamics Study

Gemini surfactants typically consist of two single-chain surfactants chemically linked by a spacer molecule. We report herein the results of fully atomistic molecular dynamics (MD) simulations of a series of Gemini surfactants: CsH2s-alpha,omega-bis(C12H25N+(CH3)2Cl-), at the air/water interface with s = 3, 4, 6, 12, 14, and 16, at values of the initial surface area per surfactant AS = 70 A2, 77 A2, 95 A2, 151 A2, 133 A2, and 103 A2, respectively. The AS values employed were obtained from surface tension and neutron reflection experiments at the respective cmc of each surfactant. The Gemini surfactant corresponding to s = 3 was also simulated at AS = 105 A2, which is the experimentally derived value of surface area per surfactant at 1/10th of cmc. Only the surfactants with s = 12 and 14 and the surfactant with s = 3 at AS = 105 A2 gave a stable monolayer at the air/water interface. In other cases, we observe movement of some surfactant molecules from the air/water interface into the aqueous phase, resulting in a stable primary monolayer of surfactants at the air/water interface and a small concentration of surfactant molecules below it. The latter form aggregates, with their hydrophobic chains in the core. The density profiles along the normal to the interface are compared with the ones obtained from neutron reflection experiments. The MD simulations confirm the bending of the spacer toward the hydrophobic chains as the spacer length is increased and the spacer becomes more hydrophobic. The simulations have helped to shed light on the low-resolution picture which emerges from experimental analyses.

Comparison of morphology and mechanical properties of surfactant aggregates at water–silica and water–graphite interfaces from molecular dynamics simulations

Cationic surfactants are important for a wide range of applications, including controlled drug delivery systems, emulsifiers, and chemical mechanical polishing. It is therefore important to better understand surfactant structure and properties at the solid-liquid interface. Here, classical molecular dynamics simulations with empirical potentials are used to compare the structures and mechanical properties of cationic surfactant micelles at hydrophobic (graphite) and hydrophilic (silica) surface-water interfaces. In particular, the morphology of monolayers and bilayers of C12TAB (n-dodecyltrimethylammoniumbromide) at these interfaces, and their responses to atomic force microscopy indentation, are examined. The simulations predict that surfactant monolayers and bilayers on silica evolve into a spherical micelle structure, in agreement with theoretical models of surfactant morphology. In contrast, surfactant monolayers on graphite evolve into a hemi-cylindrical structure, in agreement with experimental findings. In the simulated indentation of the micelle/silica system, the spherical micelle breaks apart and forms a surfactant monolayer. The indentation force curve has a maximum value of 2.25 nN. On the other hand, the simulated indentation of the micelle/graphite system causes the hemi-cylindrical micelle structure to break apart and the surfactant tails to wrap around the graphite indenter. The indentation force curve has a maximum value of 13 nN.

Molecular Dynamics Simulations of Surfactant Self-Organization at a Solid−Liquid Interface

Self-organization of aqueous surfactants at a planar graphite-like surface is studied by means of coarse-grain molecular dynamics simulations. The nonionic surfactant, n-alkyl poly(ethylene oxide), and water are both represented by coarse-grain models while an implicit representation is used for the graphite surface. The observed morphology of the aggregated surfactants depends on the alkyl chain length. Surfactants with a short chain form a monolayer on the graphite surface with a thickness roughly equal to that of the alkane tail. On the other hand, longer-tail surfactants form continuous hemicylinders on the surface with diameter approximately 5.0 +/- 0.5 nm, in good agreement with experimental AFM data.

Morphology and mechanical properties of surfactant aggregates at water-silica interfaces: molecular dynamics simulations

Dilute and concentrated surfactant systems at the solid-liquid interface are examined using classical molecular dynamics simulations. Particular emphasis is placed on understanding how surfactants aggregate and form the micellar structure, how micelles change shape at high concentrations in aqueous media and in the presence of hydrophilic surfaces, and at what force this micellar structure breaks apart during indentation of micelle-covered surfaces with a proximal probe microscope tip. The specific system of interest is C12TAB (n-dodecyltrimethylammonium bromide) surfactant in an aqueous medium that is modeled with empirical potentials. The simulations predict that the micelle structure in water is compact and either spherical or elliptical in shape. In the presence of a hydrophilic surface of silica, the structure evolves into a flat elliptical shape, in agreement with experimental findings. The simulated indentation of the micelle/silica system causes the micelle to break apart at an indentation force of about 1 nN and form a surfactant monolayer. The predicted force curve is in excellent agreement with experimental measurements.

Two-dimensional Monte Carlo simulations of ionic and nonionic silane self-assembly on hydrophilic surfaces

Aqueous chemistries have recently been shown to be useful for the deposition of hydrophobic films of nonionic and cationic silanes on hydrophilic substrates for the prevention of stiction in MEMS. The Monte Carlo method is used to simulate in two dimensions the self-assembly of silane films on a hydrophilic surface. We investigate the impact of charged group in cationic silane on the overall structure of the films. We characterize the film structure with spatial pair correlations at each molecular layer of the deposited films. The simulations reveal long-range correlations for the film of cationic silanes. Based on our two-dimensional simulations, we report an average "most probable" structure for the films of nonionic and cationic silanes.